Evaluating the potential for net-negative CO2 removal via biomass storage in anoxic marine basins

Given current global emissions trajectories, humanity will require some form of net-negative CO₂ removal (CDR) to keep average warming below climate targets of 2°C or less (IPCC AR6). As part of a suite of climate mitigation solutions, deep, anoxic, hypersaline marine basins have some meaningful advantages as potential locations for biomass C sequestration. However, we are only just beginning to understand the mechanisms controlling organic matter cycling in these extreme environments. To make informed choices about CDR-related (non)-interventions in the future, we need a more mechanistic understanding of how these and other environments would respond to sudden increases in organic matter – processes with relevance across Earth history.

To organize this interdisciplinary challenge, I will first share a conceptual framework that defines key processes across the life cycle of a hypothetical biomass deployment and considers the spatial and temporal scales needed to observe them. I will then focus in on our ongoing work in the anoxic, hypersaline Orca Basin (Gulf of Mexico), which aims to better understand how different types of organic materials, including agricultural wastes, are transformed, degraded, or preserved in anoxic brine. As part of this work, we conducted 200-day, in-situ incubation experiments on benthic landers, which were paired with (ongoing) lab-based biomass incubations. We use changes in brine and seawater geochemistry in these experiments to attribute biomass breakdown to major metabolic pathways. With these data, we can constrain the expected fate of terrestrial organic carbon in Orca Basin and, more broadly, we can begin to evaluate the feasibility and risk of biomass-based marine CDR at climatically-meaningful scales.